Abstract: A method for analysing performance of a solar photovoltaic installation data indicating measured value of power generated by the photovoltaic installation receives and stores that data in a database. An expected output of the installation for at least one period of a test day using a first method is determined. The first expected output of the installation is compared with the measured value of power generated. If the expected output of the installation determined by the first method differs from the measured value by more than an acceptable tolerance, an expected output using another method is determined. If the expected output of the installation determined by the other method differs from the measured value by more than an acceptable tolerance, an under performance warning for the solar photovoltaic installation is issued.